Homer Protein Increases Activation of Ca²⁺ Sparks in Permeabilized Skeletal Muscle

Members of the Homer family of proteins are known to form multimeric complexes capable of cross-linking plasma membrane channels (e.g. metabotropic glutamate receptor) and intracellular Ca²⁺ release channels (e.g. inositol trisphosphate receptor) in neurons, which potentiates Ca²⁺ release. Recent work has demonstrated direct interaction of Homer proteins with type 1 and type 2 ryanodine receptor (RyR) isoforms. Moreover, Homer proteins have been shown to modulate RyR-dependent Ca²⁺ release in isolated channels as well as in whole cell preparations. We now show that long and short forms of Homer H1 (H1c and H1-EVH1) are potent activators of Ca²⁺ release via RyR in skeletal muscle fibers (e.g. Ca²⁺ sparks) and potent modulators of ryanodine binding to membranes enriched with RyR, with H1c being significantly more potent than H1-EVH1. Homer did not significantly alter the spatio-temporal properties of the sparks, demonstrating that Homer increases the rate of opening of RyRs, with no change in the overall RyR channel open time and amount of Ca²⁺ released during a spark. No changes in Ca²⁺ spark frequency or properties were observed using a full-length H1c with mutation in the EVH1 binding domain (H1c-G89N). One novel finding with each Homer agonist (H1c and H1-EVH1) was that in combination their actions on [3H]ryanodine binding was additive, an effect also observed for these Homer agonists in the Ca²⁺ spark studies. Finally, in Ca ²⁺ spark studies, excess H1c-G89N prevented the effects of H1c in a dominant negative manner. Taken together our results suggest that the EVH1 domain is critical for the agonist behavior on Ca²⁺ sparks and ryanodine binding, and that the coiled-coil domain, present in long but not short form Homer, confers an increase in agonist potential apparently through the multimeric association of Homer ligand.